The transferred electron device (TED) has been used extensively in oscillator and power amplifier circuits. The TED provides the oscillator function in these circuits covering the microwave frequency range of typically 1 to 100 GHz. The oscillator and power amplifier circuits are important for many applications such as radar, intrusion alarms, microwave test instruments, and a variety of communication devices. The TED is particularly useful in these types of applications because of the solid state nature of the device which provides reliability and low cost, two features which are critical to the practical implementation of the electronic functions.
The solid state TED generates coherent microwave output power because the device has a negative differential conductance. The TED is typically comprised of doped semiconductor material which has a voltage applied that generates an electric field across a region of the semiconductor material. The voltage and width of the region are such that the electric field exceeds a critical value which is dependant on the type of semiconductor material used. The negative differential conductance is based on the phenomena that when carriers in the conduction band of the TED are accelerated by this electric field they undergo collisions with the lattice structure of the semiconductor material. These collisions transfer the carrier from a conduction band having a first energy level to a conduction band having a second, and higher, energy level. As carriers make the transition from the low energy conduction band to the higher energy conduction band in which carriers have a higher effective mass, their velocity is decreased with respect to the carriers which have not undergone the transition. As a result, carriers build up within the TED forming a charge dipole and decreasing current flow through the TED. The electric field applied to the TED then sweeps the charge dipole out of the TED which temporarily increases the current density for the applied electric field. Once the charge dipole has been swept out, the cycle of building up the charge dipole and sweeping it out begins again. The temporary decrease in current density is periodic in nature, and produces the oscillating behavior of the TED.
Conventional TEDs have been formed from a variety of semiconductor materials including GaAs, InP, CdTe, InAs, Ge, InSb, ZnSe, GaInSb, InAsP, and GainAsP. Typically, these materials in bulk form are placed between contacts to form the TED. The problem with forming a TED from these types of materials is that these materials are difficult to integrate with conventional electronic devices formed in silicon. In particular, when such materials are deposited on silicon, by a process such as chemical vapor deposition, the resulting films typically have numerous defects due to the lattice mismatch between the deposited films and silicon. The numerous defects preclude the formation of the TED useless. Also, when the TEDs are formed in a bulk material and then manually bonded to a silicon substrate, the integration of the TED and the silicon integrated circuit is expensive because of the manual labor cost. The manual placement of the TED on the silicon integrated circuit is also not desirable because of the low level of integration which can be achieved by manual placement of the TED. Moreover, a TED fabricated in bulk material requires a relatively high applied voltage in order to form an electric field having a sufficient magnitude to create the negative differential conductance effect. This is because the TED device must be very thick to accomodate the doped regions associated with the TED contacts. A high applied voltage is not desirable for a TED integrated in a silicon circuit because the silicon circuits typically use a low applied voltage.